feat(mysql): initial stable release v0.9.0

database/sql-backed MySQL client with launcher lifecycle, health check, unit-of-work via context injection, and structured error mapping.

What's included:
- Executor / Tx / Client / Component interfaces using database/sql native types (sql.Result, *sql.Rows, *sql.Row)
- Tx.Commit() / Tx.Rollback() without ctx, matching the honest database/sql contract
- New(logger, cfg) constructor; *sql.DB opened in OnInit
- Config struct with env-tag support for all pool tuning parameters
- UnitOfWork via context injection; GetExecutor(ctx) returns active *sql.Tx or *sql.DB
- HandleError mapping MySQLError.Number to xerrors codes (1062 → AlreadyExists, 1216/1217/1451/1452 → InvalidInput, ErrNoRows → NotFound)
- Driver imported as mysqldrv alias to avoid package name collision
- health.Checkable at LevelCritical; HealthCheck delegates to db.PingContext

Tested-via: todo-api POC integration
Reviewed-against: docs/adr/

docs/adr/ADR-003-driver-alias-name-collision.md

@@ -0,0 +1,47 @@
+# ADR-003: Driver Import Alias to Avoid Name Collision
+
+**Status:** Accepted
+**Date:** 2026-03-18
+
+## Context
+
+The `errors.go` file in the `mysql` package uses the `go-sql-driver/mysql` package to type-assert MySQL error values:
+
+```go
+import "github.com/go-sql-driver/mysql"
+```
+
+The Go package name of `github.com/go-sql-driver/mysql` is also `mysql` — the same as the package being authored. This creates an ambiguous identifier: within `errors.go`, unqualified references to `mysql` would be interpreted as the current package, and the driver's `MySQLError` type would be inaccessible without a disambiguating qualifier.
+
+## Decision
+
+The driver is imported under the alias `mysqldrv`:
+
+```go
+import mysqldrv "github.com/go-sql-driver/mysql"
+```
+
+In `mysql.go`, the driver is imported for its side effect only (registering itself with `database/sql`) using the blank identifier:
+
+```go
+import _ "github.com/go-sql-driver/mysql" // register driver
+```
+
+The alias `mysqldrv` is used exclusively in `errors.go`, where `MySQLError` must be referenced by name for type assertion via `errors.As`:
+
+```go
+var mysqlErr *mysqldrv.MySQLError
+if errors.As(err, &mysqlErr) {
+    switch mysqlErr.Number { ... }
+}
+```
+
+The alias is chosen to be recognisable — `drv` is a conventional abbreviation for "driver" — while making the boundary between package code and driver code immediately apparent at each call site.
+
+## Consequences
+
+- **Positive**: No ambiguity between the `mysql` package and the `mysql` driver. The alias makes the distinction explicit at every use site.
+- **Positive**: The blank import in `mysql.go` documents clearly that the driver is needed only for side-effect registration.
+- **Positive**: If the driver package is ever replaced (e.g., with a fork or an alternative driver), only the import alias and the `mysqlErr.Number` switch need to change — the rest of the package is unaffected.
+- **Negative**: Readers unfamiliar with the convention must understand that `mysqldrv.MySQLError` refers to the external driver, not the current package. The alias name and the import comment mitigate this.
+- **Note**: Error number constants (1062, 1216, 1217, 1451, 1452) are not exported by `go-sql-driver/mysql`, so they are used as integer literals with inline comments identifying the MySQL error name (e.g., `// ER_DUP_ENTRY`).